There is an increasing desire to reduce power consumption of network devices. One known way of reducing power consumption is to put a device into a mode in which it has lower power consumption than when it is in its normal operating mode. For example an interface of a network node may adopt a power saving mode when if it detects that the traffic level over the link(s) from the interface has fallen to a threshold level, or has been below a threshold level for a predefined time period. In a power saving operating mode the interface may perform operations at a lower speed than in its normal operating mode, or may even enter a “sleep mode” in which the interface becomes dormant. For example the IEEE 802.3 az standard defines a low power mode for an Ethernet interface, in which:                For 100 Mbit/s and gigabit links, an Ethernet interface that does not have data to send would put the physical layer of the system into a sleep mode. The node may signal to other nodes of the network that its interface is entering the sleep node, and other nodes may also out their interface into a sleep mode once they learn that no data will be coming from the node that has entered the sleep mode.        For 10 Gbit/s links, the operating speed is stepped down to slower speeds saving 10-20 W per link. Running at lower speed may not save as much power as putting the node into a sleep mode, but for a link operating at this speed it might take too long to wake up a sleeping interface if it was put all the way to sleep.        
An Ethernet node that operates according to the IEEE 802.3 az standard is often referred to as an “Energy Efficient Ethernet” node.
The IEEE 802.3 az standard has defined number of parameters, that are periodically transmitted over a link during the low-power state to allow the peering node or neighbour node (that is the node at the other end of the link) to refresh timing and equalization parameters during low power idle modes. The low power idle (LPI) mode in the IEEE 802.3 az standard allows for transmission of (currently) three types of frames: sleep, refresh, awake (for example, see http://www.ieee802.org/3/az/public/may08/taich_02_0508.pdf). These are very simple control words transmitted on the network and processed by the LPI circuit. A “sleep” message indicates that the interface sending the message is entering a sleep mode or other low power mode, an “awake” message indicates that the interface sending the message is leaving the sleep or other low power mode and has returned to its normal operation mode, and a “refresh” message is sent periodically by the interface while it is in the sleep or other low power mode to indicate that it is still active and to allow the partner node to refresh timing and equalization parameters.
One problem encountered when an interface of a node is put into a low power mode is that nodes communicate at different layers. One example of communication at different layers is shown in FIG. 1, which shows the 7 layers of the well-known OSI (Open Systems Interconnection) model. In ascending order, the layers are: a physical layer 1; a data link layer 2; a network layer 3; a transport layer 4; a session layer 5; a presentation layer 6; and an application layer 7. In most cases a network node will periodically exchange “check messages” with neighbouring nodes to tell the neighbouring nodes that the node is still operating correctly. Thus, routers will exchange “hello messages”, switches will send frames etc. These messages are sent at regular intervals for multiple protocols at layers 2-4 and above of the OSI model—but the sending of a check message by one layer is not co-ordinated with the sending of a check message by another layer. As noted above, the low power mode of the IEEE 802.3 az standard puts the interface into a low power mode at the level of the physical layer 1, but every time that a node is required to send a check message by a higher layer, for example by the data link layer 2, the network layer 3 or the transport layer 4, the interface must be taken out of the low power mode in order to send the check message. The lack of co-ordination between different layers in sending their check messages means that the interface is brought out of the low power mode every time that one of the layers above the physical layer wished to exchange check messages with other nodes, and this limits the power saving that can be achieved.
Furthermore, if a check message is not sent when expected because the interface of a node is in a low power mode, this could be taken as an indication that the node was faulty so that the Network Management System (NMS) would be informed of the failure of the node. This can lead to the NMS receiving false indications that a node has failed, when the node is in fact still operational but is in a low power mode.
Indeed, some protocols might consider that a “link down event” had occurred if they noted an interface in the low power mode. For example the VDSL2/GPON mechanism (see—http://www.ieee802.org/3/10GEPON_study/email/pdfV3kikUObAl.pdf) is based on longer timeouts than the IEEE 802.3 az standard and does not send refresh messages. With a VDSL2/GPON mechanism, it is possible that the OSPF (Open Shortest Path First) layer would consider an interface in the low power mode as a “link down event”.